Rechargeable galvanic cells comprise a cathode, a zinc anode, a separator having at least one layer of a semi permeable membrane and an aqueous alkaline electrolyte, such as an aqueous solution of potassium hydroxide. The cathode may comprise manganese dioxide, hydrogen recombination catalysts, hydrogen absorbers, or an air electrode. Graphite and/or carbon black is admixed to the cathode materials to provide electronic conductivity and alkaline electrolyte is admixed to provide ionic conductivity. The zinc anode mixture will include zinc or a zinc alloy as one of the main constituents, and will also include electrolyte and other constituents in known manner. These cells display superior electrical performance, in particular at high discharge rates and at low temperatures, and are widely used in many applications.
In contrast to rechargeable galvanic cells, primary galvanic cells are only discharged once and then discarded. Therefore, the performance requirements or expectations of primary and rechargeable cells are fundamentally distinct. Primary cells are expected to exhibit low self-discharge rates and satisfy demanding performance requirements. Rechargeable cells, on the other hand, are expected to demonstrate good cycle life and cumulative performance. Both types are expected to show low rates of gassing, however, the mechanisms affecting gassing are very different in rechargeable cells as gassing is measured over many recharge cycles, which alters the states of the electrodes many times. The recharge process of zinc electrodes is particularly troublesome due to zinc redistribution and the high solubility of the zinc electrode discharge product in strong alkaline electrolytes. These factors contribute to or cause observed shape changes, poorer cycle life, gassing and formation of dendrites. As a result, it has been very difficult to produce sealed rechargeable cells with zinc electrodes without providing a resealable venting mechanism that would release excessive gassing during cycling and storage. It would be, therefore, not feasible to attempt to predict the effect of a change in, for example, electrode make-up, on the performance of a rechargeable cell from the effect of such a change on the performance of primary cells.
Because of environmental concerns regarding disposal of batteries, toxic additives in manganese/zinc cells such as mercury and lead are being drastically reduced or eliminated from the cells. U.S. Pat. No. 5,626,988 describes in its background how the addition of mercury provides inhibition of zinc corrosion, resultant hydrogen gassing and electrolyte leakage. It also describes how mercury provides conductivity in the anode resulting in superior electrical performance, in particular at high discharge rates, at low temperature and under conditions where the cells are exposed to mechanical shock and vibration. It further describes the use of surfactants and various metals including indium, for inhibiting corrosion and preventing dendrite formation in rechargeable cells.
Also described is known art relating to primary or single use galvanic cells regarding the problem of the surface coating of zinc powders with appropriate metals or their compounds, prior to processing the negative electrode, many of the techniques being complicated and frequently requiring washing and drying steps.
U.S. Pat. No. 5,626,988 further describes a sealed rechargeable cell containing a mercury-free zinc anode and a method of manufacture, which includes treating a zinc or zinc alloy powder with indium sulfate, and more particularly with both an organic surfactant and electrolyte. The zinc powder is coated with a surfactant, and separately with an acidic aqueous solution of indium sulfate. Without any subsequent filtering, washing or drying, the powder is mixed with electrolyte and gelling agent and assembled into the cell.